An object containing radium was recently found underneath a house on San Francisco‘s Treasure Island – and now the U.S. Navy is preparing to test hundreds of homes on the island for radiation. Although officials believe there are no health hazards, they decided that a radiological survey should be conducted inside and underneath housing units in the area.

Treasure Island is a man-made landform that was originally built for the 1939 Golden Gate International Exposition. During World War II the site was turned into a navel base that operated from 1941 to 1997. The piece of radioactive material was found on land occupied by the former military base, and it’s the latest in a string of radiological discoveries in the last seven years.

The base hosted sailor training sites, administrative areas and military factories – which sometimes use radioactive materials. The navy began cleaning up the site in 1997 and the island has been redeveloped for residential housing – although fragments of its past still linger. Around 8,000 residential units are still planned for construction on Treasure Island, which currently houses 1,500 San Francisco residents. Handheld devices will be used to scan the ground floors of homes in the area, and if any radiation is detected safety measures will automatically be deployed.